IBM 1 Series - Chapter 3. Timer Feature; Introduction

Introduction
Chapter 3. Timer Feature
The timer feature provides two 16-bit timers: timer 0 and timer 1 (Figure
3-1). Each timer can be used as an interval timer, a pulse counter, or a
pulse duration counter with end interrupt. The timers are packaged on one
printed circuit card, which plugs into a backpanel that distributes the
processor I/O channel signal lines. Electrical power for the timer circuitry
is obtained from the backpanel through card-connector pins.
The timer feature also has a 16-pin connector that allows the timers to be
used with external signals. Each timer has two output lines, ‘run state’ and
‘external gate enable,’ and two input lines, ‘customer clock’ and ‘external
gate.’ Drivers and receivers for these lines are TTL-compatible.
The two timers are separately addressable and are independently started,
stopped, read, or set to any value under program control. The timers can
be read without disturbing their operation; however, to set the timer’s
value or mode, it must be stopped.
Each timer also has a mode register that is used to select one of four
internal time bases or an external time base. The internal time bases are 1,
5, 25, and 50 microseconds. The external time base is provided by the user
and must be equal to or greater than 20 microseconds when the input is
filtered, or 1 microsecond when not filtered. The timer value is
decremented with the selected time base. For additional information, see
“Receivers” in this chapter. An external gate enable bit is also contained
in the mode register. Both the time base and the external gate enable bit
are program-selectable.
The program-selectable running modes available for each timer are:
e Periodic interrupts—internal. A 16-bit auto-load register is set to any
value by program control. This register automatically reloads the timer
when the timer underflows, and causes an interrupt to be generated.
This allows periodic interrupts to be generated on 65,536 possible base
values of the timer without program intervention.
e Aperiodic interrupts—internal. The timer is loaded with a value, under
program control, and an interrupt occurs when the timer underflows.
After the first interrupt, the timer is not reloaded from the auto-load
register and, therefore, counts the full 65,536 counts before the next
interrupt occurs, unless a new value is loaded, under program control.
e Periodic or aperiodic interrupts—external. The timer generates periodic
or aperiodic interrupts, but timer start and stop is controlled by the
external gate when the timer is in the run state.
Timer Feature 3-1